Motors are used in various appliances to convert electrical power into mechanical energy to produce actuation about an axis. For example, in a washing machine a motor can be coupled with a washing tub such that the tub rotates at various speeds. In an air conditioner, a motor can be coupled with a fan to circulate air. In a refrigerator, a motor can be coupled with a compressor to supply cool air into a fresh food and/or freezer compartment.
A permanent magnet synchronous machine (PMSM) motor can include a rotor having a permanent magnet and a stator having a plurality of electromagnetic poles such that the rotor rotates about an axis by locking into the rotating magnetic fields generated on the stator. In a PMSM motor, base speed is the maximum speed the motor can reach without modifying the magnetic field between a rotor and stator of the motor. If speeds above the base speed are desired, the motor can be controlled using field weakening techniques. Field weakening can reduce the magnitude of the magnetic field (magnetic flux) between the rotor and the stator, thereby reducing torque and increasing the speed of the motor above the base speed without causing damage to the motor.
Traditionally, field orientated control (vector control) can be used to control a motor. In vector control systems, the voltage applied to the stator poles is adjusted to achieve the desired speed. When a speed greater than the base speed is desired, field weakening is achieved by modifying a voltage to have a constant maximum magnitude and calculating the phase angle using a micro-processor. However, the implementation of this technique is inefficient due to limited memory space in the micro-processor and a reduction in computational speed. In addition, calculation of the phase angle includes significant calculations such as detecting the current in AC, transforming the AC current signal into a DC signal, independently controlling a real and imaginary component of the DC signal to modify the voltage, and transforming the DC signal back into an AC signal. This calculation requires significant micro-processor resources and leads to complexity of the system.
Thus, a need exists for an improved field weakening control technique of a PMSM motor that can achieve field weakening using reduced computational power to achieve a desired speed.